The present invention relates to a synchronizing signal generating circuit used in an optical scanning device, and specifically, to an optical scanning device for emitting a plurality of laser beams which scan simultaneously. Optical scanning devices are typically included in laser beam printers, facsimile devices, or the like.
In a conventional optical scanning device, a plurality of laser beams are emitted from a light source (for example, a plurality of laser diodes) and are deflected and scanned in a predetermined angular range (deflection angle) and direction by a rotating polygonal mirror. The laser beams pass through an f.theta. lens that causes the laser beams to scan at a constant speed along scan paths, which are parallel, in a predetermined scan plane. The movement of the laser beams through the deflection angle in the scan plane is referred to as a main scan.
If the optical scanning device is used in, for example, a laser beam printer, a uniformly charged photoconductive surface (light receiving surface) of a rotatable photoconductive drum is the target to be scanned. During each main scan, the laser beams pass along the photoconductive surface. A predetermined image area on the photoconductive surface is scanned by the laser beams which are optically modulated (i.e. turned on and off), respectively, according to image data that is received from an external source, such as a personal computer. The photoconductive drum is rotated in synchronization with the main scan. The rotation of the photoconductive drum is referred to as a sub scan. When the laser beams hit the uniformly charged photoconductive surface they form an electrostatic latent image, corresponding to the image data received, on the photoconductive surface.
As the photoconductive drum is rotated, toner is adhered to the electrostatic latent image, in a pattern corresponding to the electrostatic latent image, forming a toner image. Then, as the photoconductive drum is further rotated, the toner image is transferred to and fixed on a recording sheet.
The image area is delineated by an image start position and an image end position for each laser beam that are located near opposite ends of the photoconductive drum. A synchronizing signal H.sub.SYNC (horizontal synchronizing signal) is used to synchronize the image start position so that it is the same for each laser beam and for each main scan. That is, the synchronizing signal H.sub.SYNC synchronizes the position at which the image data transfer starts for each laser beam, and thus, synchronizes the position on the recording paper at which the toner image transfer starts. It is important that the image start position remain consistent for each laser beam and for each main scan in order to produce accurate printing.
In a conventional optical scanning device, a photodiode is positioned at a synchronizing position that is on the main scan path but outside the image area at a predetermined distance from the image start positions. The laser beams are turned on while passing over the synchronizing position such that the photodiode outputs a signal. The synchronizing signal H.sub.SYNC is generated based on the signal output from the photodiode and the start positions for the image area are those positions reached by the laser beams a predetermined amount of time after the synchronizing signal H.sub.SYNC.
In conventional optical scanning devices which emit a plurality of scanning beams and which use a photodiode to detect the laser beams, it is difficult to generate the synchronizing signal H.sub.SYNC for each laser beam accurately.